For me it depends on my mood and the moment. Sometimes it takes a while to enter into a meditative state, other times I can enter quickly. I often just feel when to stop, but if I have something to do after, I set an alarm. For example for 20 minutes. Depending on how much time I've got.
I have heard from those who have practiced a long time that even one minute can help. I think you have to work up to what is right for you. I would say at least 5 minutes really makes a difference. I’m a beginner who finds 10 minutes fine while 15 minutes seems a little long.